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Slip Sliding Away

03.12.04

On February 9, the wake up song for Opportunity was "Slip Sliding Away" by
Simon and Garfunkel in honor of the experience the prior day when Opportunity
first tried to scoot up the crater slope at Meridiani Planum. The ambitious
rover aimed for its target and willingly went forth up the slope; nevertheless,
Opportunity slipped and slid in the sand, making it only half way to
its target.

But ignorance is bliss, and while Opportunity's
wheels had a hard time gaining traction on
the sandy surface, the rover's brain (or computer)
had a hard time grasping that it hadn't successfully
made it to its target. The little rover didn't
have to encourage itself to make it up the
crater slope later by chanting, "I think I
can, I think I can," because the little rover
thought it actually did make it up the slope
the first time.

Image left: Opportunity
failed to complete the first big climb of the
outcrop on February 8 due to the wheels slipping
up the martian slope. Credit: NASA/JPL.

Measuring the Distance Traveled
on Mars

"Like a car on Earth, each Mars rover uses
an odometer to click off the distance its wheels
travel to measure and register how far the
vehicle has moved," explains Randy Lindemann,
rover mobility lead. One revolution of the
rover wheel equals 80 centimeters or 2.6 feet,
so after the wheels have revolved four times,
the rover believes that it has moved forward
320 centimeters or 10.5 feet (80 centimeters
X 4 = 320 centimeters).

Image right: Randy
Lindemann, Rover Mobility LeadCredit: NASA/JPL.

Alas, Opportunity's wheels had a hard time
grasping onto the sandy ground around the crater
and the wheels spun in place before they actually
gained tracking. "As Opportunity's wheels turned
and ticked off 80 centimeters (2.6 feet) each
revolution, they eventually spun four times,
calculating to what it thought was a distance
of 320 centimeters (10.5 feet). Thus, Opportunity
believed it had reached its goal, when in reality,
it had spun in place 50 percent of the revolutions
and only really made it 160 centimeters (5.25
feet)," said Lindemann.

Outwitting the Odometer

In order to prevent any future missed targets,
Opportunity's mobility experts quickly started
trying to predict exactly how far the rover
would slip down a slope or fall short of a
target while climbing up a slope due to the
loose terrain along the steep angles of the
crater wall. "Since the rover isn't on cruise
control and can't rev its engine to get some
extra oompf to go up a hill, we continually
have to outwit Opportunity's odometer and command
the rover to go farther or shorter than the
real target distance," said Rover Driver Eric
Baumgartner.